Members of this family of plant pathogenic proteins adopt an elongated structure somewhat reminiscent of a mushroom that can be divided into 'stalk' and 'head' subdomains. The stalk subdomain is composed of the N-terminal helix (alpha1) and beta strands beta3-beta4. An antiparallel beta sheet (beta5, beta7-beta8) forms the base of the head subdomain that interacts with the stalk. A pair of twisted antiparallel beta sheets (beta1 and beta6; beta2 and beta9/9') supported by alpha2 form the dome of the head. The head subdomain possesses weak structural similarity with the catalytic portion of a number of ADP-ribosyltransferase toxins.